The present invention relates to a molecular single-crystalline film (which herein refers to a film having a thickness of at most ca. 100 μm and having a portion which retains a single crystal state having a uniform molecular crystalline alignment over the thickness and over an areal extension including a side length of at least 10 times the thickness, i.e., an areal size useful as a functional film, preferably an areal size of at least 50 μm×50 μm) and a process for the production thereof.
A molecular crystal can be expected to be a useful device material, such as a superconducting material, an effective photoconductor or a gas sensor, because of its electrical and geometrical structure and packing state. As the process for the production thereof, growth in a solution and growth in a molten state have been generally practiced. According to any of such processes, however, it is difficult to obtain a thin film of single crystal by suppressing an increase in thickness, and this poses an obstacle against using it as a functional layer in devices which have a laminar structure in many cases. As another process, there is known a gas phase deposition process, by which, however, it is difficult to prepare a uniform film due to affection by the gas phase deposition boundary.
On the other hand, carrier transportation performance has been reportedly improved by utilizing a molecular alignment in a higher order liquid crystal phase of SmB or SmE (Ohyou Butsuri, Appl. Phys., vol. 68, no. 1, pp. 26-32 (1999)). In this report, a higher speed transportation of electrons and holes has been aimed at by utilization of alignment order in a higher order liquid crystal phase. The improvement in high speed transportation performance has been considered attributable to the formation of flow paths for electrons and holes due to regular packing of aromatic rings in the higher order smectic phase alignment. This performance has been also noted as a carrier transportation layer in EL devices, and further improvement is expected.
Regardless of whether it is a liquid crystal or a (solid) crystal (herein a term “crystal” without further notation is used to mean a solid crystal), the film thereof is required to assume a single crystal state free from defects (i.e., free from carrier traps) in order to function as a functional layer as mentioned above.
Then, if a (solid) single-crystalline film can be obtained, it is expected to achieve a higher speed and higher density carrier transportation because of a higher degree of order and a closer packing of molecules than a liquid crystal film.